The present invention relates to a plunger piston assembly or system used, for example, to support and guide a pneumatic spring bellows of an air suspension axle of a commercial vehicle or the like, and including a plunger piston that has a cylindrical plunger piston skirt with a bottom foot region for a connection with an air spring bearing arm and an upper plunger piston edge that adjoins the plunger piston skirt and which becomes, radially in an inward direction, a trough section that has a trough base that incorporates an opening for the passage of a bolt for a force-fit connection with a base of the air spring bellows, the trough section forming a trough to accommodate a convex base of the air spring bellows. A supporting body within an inner space of the piston is surrounded by the plunger piston skirt and has a lower foot area on the bottom foot area of the plunger piston.
More and more frequently, commercial vehicles are being equipped with air spring systems to increase the level of comfort provided thereby, to take advantage of the height adjustment and height control, to ensure optimal distribution of axle loads by means of compensation lines in multi-axle units, and to protect the surface of the road. Air spring axles within multi-axle assemblies can be raised in a simple manner by auxiliary systems in order to protect the tires.
Pneumatic springs can transfer only vertical forces. Other chassis elements are required in order to absorb all the other forces and moments. Known air suspension systems comprise primarily an air spring bellows that frequently includes a rubber buffer as a stop, a plunger piston, and a bearing arm that is, in turn, mounted on or under the vehicle. The air spring bellows is in the form of a rotationally symmetrical rubber sack that can be filled with air and that is connected to the vehicle frame by means of a steel plate that is fastened at the top as a force transfer element. A round steel base that can support the rubber buffer inside the air spring is clamped or vulcanized to the bottom of the bellows. The base has a convex underside that is a form fit in a correspondingly concave upper trough of the plunger piston and is securely bolted thereto.
In the usual steel version, the plunger piston is a rotationally symmetrical deep-drawn or extruded part, the surface of which is provided with corrosion protection. Its skirt, which is essentially cylindrical, is shaped so as to be slightly conical to the outside and then flanged inward in a semi-circle. Holes in the flanged edge facilitate connection to the plate-shaped end of the air spring bearing arm. In the upper section, the plunger piston skirt makes the transformation through an essentially semi-circular edge into a truncated conical trough with a level bottom, this serving to accommodate the convex trough of the air spring bellows. The base of the trough incorporates at least one drilled hole through which a screw bolt can be passed to form a friction fit connection between the plunger piston and the air spring bellows. When under compression, the base of the air filled spring bellows is friction fit in the trough of the plunger piston, whereas the side wall of the essentially cylindrical air spring bellows is slipped over the upper edge of the plunger piston and the essentially cylindrical plunger piston skirt. The plunger piston can move so deep into the air spring bellows that the rubber buffer in the air spring bellows is clamped between the upper steel plate and the plunger piston of the air spring bellows, thus forming an end stop. In such cases, shock loads exert a significant amount of stress directly on the surface of the trough base.
Because they are made of metal, such plunger pistons are relatively heavy and costly. Furthermore, metal plunger pistons can corrode in their rolling region after prolonged periods of use, and after the corrosion protection has worn off increased wear of the rubber sack can occur. Therefore, attempts have been made to develop a plunger piston of glass-fiber reinforced plastic, so as to reduce weight and production costs, as well as to increase the useful life of the entire air spring bellows, as a result of the smooth and corrosion-free surface of the plunger piston. In this regard, the outer shape of the plunger piston is similar to that of the embodiment that is of steel. However, the edge near the foot is not flanged, but its cross-section is approximately trapezoidal so as to ensure improved seating on the bearing arm plate. Perpendicular to the foot edge at most four ribs that are offset by 90.degree. relative to each other extend along the inner surface of the skirt up to the upper edge of the plunger piston. In their lower section, the reinforcing ribs incorporate threaded holes or threaded inserts of metal, these being used for the bolted connection to the bearing arm plate.
In a further embodiment of a plunger piston of glass-fiber reinforced plastic, it has been proposed to incorporate a plurality of reinforcing ribs internally in the area of the upper rounded transition from the plunger piston skirt to the truncated conical trough. In the event of shock stresses however, breaks still occur at the upper stop at the transition from the plunger piston skirt to the truncated conical trough. On the other hand, it has also been seen that the base of the trough has been torn away on rebound, because the effective tensile force is transferred through the screw head and, optionally, a washer, from the air spring bellows directly to the trough base of the plunger piston. Besides these functional disadvantages, because of the numerous ribs, this embodiment of the plunger piston is almost as heavy as the steel version. Furthermore, the tool costs associated with the production of such embodiments of plunger pistons are extremely high.
In still another embodiment of a plunger piston of glass-fiber reinforced plastic, a pipe stub is molded into the interior of the piston, starting concentrically from the trough base. This pipe stub extends downward to the supporting arm plate and rests on such plate when subjected to a load. The object of this pipe stub is to transfer shocks from the base of the trough to the bearing arm plate and to remove the load from the edge area of the plunger piston. However, the disadvantage of this embodiment is that in the production process that is used, the reinforcing glass fibers do not get into the lower third of the pipe stub, so that it fails under elevated shock stresses. In addition, the dimensions of the pipe stub make it difficult to install the supporting arm plate on a parabolic link, which is necessary under some circumstances, e.g. when for structural reasons it should be so installed instead of on the supporting arm. For this reason, an area on the bottom edge of the short pipe has been subsequently notched, which of course further reduces the supporting effect.
GB-PS 1 231 766 describes a damped pneumatic spring with main and secondary chambers connected by an opening in a partition wall that separates them. The main chamber is formed by an air spring bellows, the base of which is connected with the base plate of a plunger piston that forms the second chamber that is closed up to the connection with the first chamber. The upper base plate of the plunger piston is welded to a lower cover plate through a pipe-like member. The plunger piston wall is formed in part form a wall section that is sharply curved and adjacent to the upper base plate, and in part from a flange area that slopes upwards from the lower cover plate. If this plunger piston system is to be used for supporting and guiding an air spring bellows incorporated in an air-sprung axle of a commercial vehicle or the like, the plunger piston and the pipe-like element must have extraordinarily thick walls in order to be able to withstand the stresses to which they will be subjected. Because of its construction, this know plunger piston arrangement is very heavy and entails very high production costs.